/*
 * Copyright 2002-2017 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.web.server.adapter;

import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.ServletException;
import javax.servlet.ServletRegistration;

import org.springframework.context.ApplicationContext;
import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.http.server.reactive.HttpHandler;
import org.springframework.http.server.reactive.ServletHttpHandlerAdapter;
import org.springframework.util.Assert;
import org.springframework.web.WebApplicationInitializer;

/**
 * Base class for a {@link org.springframework.web.WebApplicationInitializer}
 * that installs a Spring Reactive Web Application on a Servlet container.
 *
 * <p>Spring configuration is loaded and given to
 * {@link WebHttpHandlerBuilder#applicationContext WebHttpHandlerBuilder}
 * which scans the context looking for specific beans and creates a reactive
 * {@link HttpHandler}. The resulting handler is installed as a Servlet through
 * the {@link ServletHttpHandlerAdapter}.
 *
 * @author Rossen Stoyanchev
 * @since 5.0.2
 */
public abstract class AbstractReactiveWebInitializer implements WebApplicationInitializer {

    /**
     * The default servlet name to use. See {@link #getServletName}.
     */
    public static final String DEFAULT_SERVLET_NAME = "http-handler-adapter";


    @Override
    public void onStartup(ServletContext servletContext) throws ServletException {
        String servletName = getServletName();
        Assert.hasLength(servletName, "getServletName() must not return empty or null");

        ApplicationContext applicationContext = createApplicationContext();
        Assert.notNull(applicationContext, "createApplicationContext() must not return null.");

        refreshApplicationContext(applicationContext);
        registerCloseListener(servletContext, applicationContext);

        HttpHandler httpHandler = WebHttpHandlerBuilder.applicationContext(applicationContext).build();
        ServletHttpHandlerAdapter servlet = new ServletHttpHandlerAdapter(httpHandler);

        ServletRegistration.Dynamic registration = servletContext.addServlet(servletName, servlet);
        Assert.notNull(registration, "Failed to register servlet '" + servletName + "'.");

        registration.setLoadOnStartup(1);
        registration.addMapping(getServletMapping());
        registration.setAsyncSupported(true);
    }

    /**
     * Return the name to use to register the {@link ServletHttpHandlerAdapter}.
     * <p>By default this is {@link #DEFAULT_SERVLET_NAME}.
     */
    protected String getServletName() {
        return DEFAULT_SERVLET_NAME;
    }

    /**
     * Return the Spring configuration that contains application beans including
     * the ones detected by {@link WebHttpHandlerBuilder#applicationContext}.
     */
    protected ApplicationContext createApplicationContext() {
        AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();
        Class<?>[] configClasses = getConfigClasses();
        Assert.notEmpty(configClasses, "No Spring configuration provided.");
        context.register(configClasses);
        return context;
    }

    /**
     * Specify {@link org.springframework.context.annotation.Configuration @Configuration}
     * and/or {@link org.springframework.stereotype.Component @Component}
     * classes that make up the application configuration. The config classes
     * are given to {@linkplain #createApplicationContext()}.
     */
    protected abstract Class<?>[] getConfigClasses();

    /**
     * Refresh the given application context, if necessary.
     */
    protected void refreshApplicationContext(ApplicationContext context) {
        if (context instanceof ConfigurableApplicationContext) {
            ConfigurableApplicationContext cac = (ConfigurableApplicationContext) context;
            if (!cac.isActive()) {
                cac.refresh();
            }
        }
    }

    /**
     * Register a {@link ServletContextListener} that closes the given
     * application context when the servlet context is destroyed.
     *
     * @param servletContext     the servlet context to listen to
     * @param applicationContext the application context that is to be
     *                           closed when {@code servletContext} is destroyed
     */
    protected void registerCloseListener(ServletContext servletContext, ApplicationContext applicationContext) {
        if (applicationContext instanceof ConfigurableApplicationContext) {
            ConfigurableApplicationContext cac = (ConfigurableApplicationContext) applicationContext;
            ServletContextDestroyedListener listener = new ServletContextDestroyedListener(cac);
            servletContext.addListener(listener);
        }
    }

    /**
     * Return the Servlet mapping to use. Only the default Servlet mapping '/'
     * and path-based Servlet mappings such as '/api/*' are supported.
     * <p>By default this is set to '/'.
     */
    protected String getServletMapping() {
        return "/";
    }


    private static class ServletContextDestroyedListener implements ServletContextListener {

        private final ConfigurableApplicationContext applicationContext;

        public ServletContextDestroyedListener(ConfigurableApplicationContext applicationContext) {
            this.applicationContext = applicationContext;
        }

        @Override
        public void contextInitialized(ServletContextEvent sce) {
        }

        @Override
        public void contextDestroyed(ServletContextEvent sce) {
            this.applicationContext.close();
        }
    }

}
